Axons travel frequently in bundles to reach their target. Upon arriving at the target, axon terminals defasciculate, migrate to topographicaJly defined positions, and form synapses with appropriate target neurons. Molecular signals regulating these processes are not well understood. Our previous studies indicate that members of the Eph family tyrosine kinase receptors, and their ligands, the ephdns, play critical roles in regulating hippocampat projections. The Eph receptors consist of EphA and EphB subfamilies. Our studies show that the EphA receptors and ligands are expressed in opposing gradients in the hippocampus and its major subcortical target, the lateral septum, respectively, and interactions between the receptor and the ligand gradients define topographic positions of hippocampal axon terminals. In contrast, our preliminary studies showed that a member of the EphB-type receptors, EphB2, is expressed uniformly in the hippocampus, and a ligand, ephrin-B3, is transcribed uniformly in the septal target. Exposure of ephrin-B3 leads to the dispersion of hippocampal neurons and defasciculation of hippocampal axons. Mice missing the EphB2 receptor show abnormal axon bundling in the septum. These observations tend to the hypothesis that EphA and EphB receptors and their cognate ligands are functionally distinct in regulating hippocampal axon targeting to the lateral septum, with the A-type as topographic mapping tags and the B-type as modulators of axon defasciculation. To test this hypothesis, we propose to (1) Elucidate the spatial and temporal expression of EphB receptors and ligands. (2) Delineate effects of B-ephdns on hippocampal axons in vitro. (3) Analyze effects of inactivation of EphB receptors and ligands on hippocampal axon defasciculation in vivo. (4) Examine the molecular mechanisms of Eph receptor functions. The proposed studies will provide new insights into the molecular mechanisms of axon guidance, which may facilitate development of strategies to regenerate neural circuits after injuries or in diseases such as Alzheimer's and Parkinson's diseases.